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The chemical composition and structure of adsorbed material on pore surfaces in Middle East reservoir rocks Kion Norrman, Theis Ivan Sølling, Marcel Ceccato, Eugen Stamate, N. Bovet, and S. L. S. Stipp Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02422 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 3, 2018
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The chemical composition and structure of adsorbed material on pore surfaces in Middle East reservoir rocks K. Norrman,*,† T. I. Sølling,‡,§ M. Ceccato,‡ E. Stamate,† N. Bovet,‡ and S. L. S. Stipp‡ †
Department of Energy Conversion and Storage, Technical University of Denmark, Frederiksborgvej 399, DK-4000 Roskilde, Denmark ‡ Department of Chemistry, Nano-Science Centre, University of Copenhagen, Universitetsparken 5, DK-2100 København Ø, Denmark § Center for Integrative Petroleum Research, College of Petroleum & Geosciences, King Fahad University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
ABSTRACT Several recent studies have shown that some skeletal limestone reservoirs are far more oil-wet than the typical, water wet, biogenic, limestone reservoirs, such as chalk. It is challenging, even with state-of-theart approaches, to completely remove the residual hydrocarbons from skeletal limestone core samples and restore the pore surfaces to the water wet conditions that are assumed to prevail before oil entered from the source rock. We used a combination of gas chromatography mass spectrometry (GC-MS), X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (TOF-SIMS) to investigate pore surface composition and we propose an explanation for the phenomena. The hydrophobic nature of pore surfaces is likely caused by adsorbed organic molecules that are strongly attached to the calcite surfaces through carboxylic functional groups and their fatty ends serve as anchors for the lipophilic oil components. The strong binding of the carboxylate to the pore surfaces explains why it is only possible to remove some of organic material using the conventional core cleaning methods, such as Soxhlet solvent extraction. Unless a solvent is able to replace the carboxylate terminated species, the rock remains oil-wet or at least mixed wet, regardless of the type or extent of the cleaning procedure. INTRODUCTION Large volumes of the world's oil resources are hosted in carbonate rock reservoirs. With the extraction methods currently used in the primary and secondary phases of oil recovery, a large fraction (in some cases more than 90%) of the oil is left in the reservoir.1 Many carbonate rock reservoirs are either strongly or moderately oil-wet. Methods have been developed to control wettability, to enhance spontaneous imbibition and to increase recovery.2–25 Wettability modification is one of the important components in enhanced oil recovery (EOR) processes16–25 so it would be useful to have a clearer understanding of the physical and chemical controls on the properties and behaviour of the solid-fluid interface and the nature of the organic molecules that adhere to the mineral surfaces.17,26 Calcite is the dominant mineral in limestone and chalk drinking water aquifers as well as oil reservoirs so modification and control of calcite surface properties, especially wettability, is also relevant for remediation of contaminated aquifers, and in industrial applications, such as for calcite powder production and scale prevention in pipes and cooling systems. While pure, clean calcite surfaces are hydrophilic, even a fraction of a monolayer of organic material makes them hydrophobic and adsorption of adventitious carbon is rapid and for the most part, irreversible.27,28 Thus, adsorption of components from crude oil can dramatically change calcite surface wettability.29
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Oil-rock-water interactions play a fundamental role in determining the amount of oil recovered but even in systems where oil is not present, such as on sediment particles in seawater, fresh water and in air, organic and inorganic compounds adsorb spontaneously, changing surface composition and affinity for other organic molecules. Previous work suggests that it is likely to be molecules with -O(H) or -COO(H) functional groups that form stronger bonds with calcite than water,30 but no study has yet specifically identified which molecules form that first layer, that is in such strong contact with the calcite surface. Clearly, better understanding of the nature of the oil components that adhere to limestone pore surfaces would provide useful insight for optimizing EOR strategies. In order to understand and thus control wettability on limestone pore surfaces, it is vital to understand the pore surface chemistry and the solution chemistry interacting with it. It therefore makes sense to employ chemical characterization techniques. In this work the oil from limestone samples were extracted by Soxhlet treatment and analyzed by traditional gas chromatography mass spectrometry (GC-MS) analysis.31 GC-MS separates the oil components and provides a mass spectrum (i.e. a mass spectral fingerprint) of each component, which makes it more or less straight forward to identify the compounds via a mass spectrometry library. It is somewhat more complicated to perform a chemical analysis directly on the pore surface since no separation technique is available. In this work two chemical surface characterization techniques were employed in order to analyze the pore surface chemistry. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS) provide complimentary useful information. TOF-SIMS32 has been used but XPS27,33 is the more well-known technique of the two when it comes to limestone surface research. Table 1 summarizes the capabilities, type of information and differences between the two techniques. Table 1. Capabilities, type of information and differences between XPS and TOF-SIMS. Parameters
XPS
TOF-SIMS
5–10 nm
1–2 nm
~1 µm
~50 nm
~0.1 atom%
ppm to ppb
Quantitative
Yes
No
Vacuum technique
Yes
Yes
Elements detected
All except H
All
Solid phase only
Solid phase only
Identifies atoms based on coreelectron binding energy. Identifies oxidation state and the identity of possible bonded neighbor atoms based on shifts in core-electron binding energy.
Identifies atoms based on exact mass and/or isotopic mass spectral patterns. Molecular structure information for organic compounds based on mass spectral fragmentation patterns.
Probe depth Image resolution Detection limit
Gas, solution or solid phase Type of information
The two chemical characterization techniques are similar in the sense that in both cases the surface is exposed to either energetic ions (TOF-SIMS) or soft X-rays (XPS), and in both cases expelled charged species from the surface are analyzed, for TOF-SIMS it is ions (molecular ions, fragment ions, cluster ions or atomic ions) and for XPS it is electrons. In a TOF-SIMS analysis atoms are identified based on exact mass (i.e. peak position) and/or mass spectral isotopic patterns. Molecular structure information for organic compounds is based on mass spectral fragmentation patterns (fore pure compounds). In an XPS 2 ACS Paragon Plus Environment
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analysis atoms are identified based on core-electron binding energies. Oxidation state and the identity of possible bonded neighbor atoms are based on shifts in core-electron binding energies. Both techniques are very surface sensitive and suffer from the same problem, which is, since no separation of compounds is available all the chemical information consequently originates from all the components on the surface (excludes library use), which is a very complex and challenging situation, especially when dealing with materials from nature, such as for example oil-wet limestone. Both techniques have imaging capabilities that provides chemical images of the surfaces. However, the imaging resolution is inferior for studying distribution of chemistry in the pore surfaces. In this work all the combined spectral chemical information that is possible to extract from oil-wet limestones has been acquired, and the data, results and conclusions are presented herein. The main goal in this work was to gain information about the chemical composition of the organic material that is present on the limestone pore surfaces. The chemical information was used to explain the oil-wet nature of the Middle East reservoirs and to shed light on what distinguishes them from their chalk counterparts. MATERIALS AND EXPERIMENT DESCRIPTION Materials and sample preparation Two types of limestone samples from the oil zone were analyzed. The samples were selected based on the highest producing and the most oil-wet reservoirs. Samples that were as flat as possible were chosen (surface analysis requirement) with the largest length being approximately 1 cm. Two subsamples of each sample type
were used, where one was fresh, untreated and stored under dry conditions, and the other was cleaned rigorously using the Soxhlet solvet extraction method;34 repeatedly flushing the sample with a mixture of 7% methanol and 93% dichloromethane until the effluent liquid was colourless. All Soxhlet treatments were performed for 48 hours. Because TOF-SIMS and XPS are highly surface sensitive techniques, they are vulnerable to surface contamination and because contamination is often organic compounds, it required particular care in sample handling. Using a similar approach as in previous studies, we fractured small pieces from the limestone samples and analyzed the freshly fractured surfaces. We assumed that the rock broke along fluid pathways because these would yield to stress more easily that fractures across crystals. The samples were mounted, put into the vacuum system and analyzed within an hour after fracture to minimize exposure to air, which would result in adventitious carbon contamination. However, adsorption of carbon compounds from air and water is unavoidable and it takes place within seconds, particularly for calcite. The samples, A and B, came from two different locations in the same reservoir. The subsamples A1 and B1 were investigated in their untreated state but for Samples A2 and B2, the associated oil was removed using Soxhlet extraction. Later analysis showed that strongly adhering species remained. It is possible that the treatment itself resulted in some adsorbed contamination by the solvents. GC–MS analysis The Soxhlet extracts were analyzed using an Agilent 5973/N 6890 GC–MS with electron ionization. The capillary column was a 30 m HP-5MS with an internal diameter of 0.25 mm and film thickness of 0.25 µm. 1 µL of extract was injected in pulsed, splitless mode at an injection temperature of 315 °C. The column was held at an initial temperature of 40 °C for 2 minutes. Subsequently, the temperature was increased at a rate of 25 °C min–1 to 100 °C and then by 5 °C min–1 to 315 °C, then held constant for 13.4 minutes. The transfer line temperature was 315 °C and the quadrupole temperature, 280 °C. The carrier 3 ACS Paragon Plus Environment
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gas was helium, at a flow rate of 1.1 mL s–1. The ion chromatograms were analyzed using the software ChemStation, provided with the instrument. XPS analysis The XPS analyses were performed on a K-alpha (Thermo Electron Limited, Winsford, UK), using a monochromatic AlKα X-ray source where the take off angle was 90° from the surface plane, which maximises information depth. A combined ion/electron gun (i.e. a dual beam source) was used to control sample charging. All samples were mounted on the sample holder, which resulted in a chamber pressure of 5×10–7 mbar. Atomic concentrations were determined from the average of 5 broad range spectra (0– 1,300 eV, collected with 200 eV detector pass energy, 50 ms dwell time) and were determined from integrating peak intensities of the characteristic peaks. The software package Thermo Avantage (v5.979, build 06465) was used and the Smart type background correction was applied. This algorithm is an integral part of the software and is based on multiple Shirley background corrections. The binding energies were referenced to the Au 4f peak (Figure S1 in Supporting Information, description of the gold sputtering procedure). Three different surface sites were analyzed on each subsample and average intensities were determined. Uncertainties reported are simply the range of the three determinations. High resolution, local binding energy spectra were sequentially obtained for C 1s, Ca 2p, O 1s, S 2p, N 1s and Au 4f, using 50 eV detector pass energy collected over 20–50 scans. Peaks were fit using a full width at half maximum (FWHM) of 1.6 eV and a 55% Lorentzian/Gaussian function. Binding energies for the C 1s peak were taken from the NIST Standard Reference Database.35 TOF-SIMS analysis We used a TOF-SIMS IV (ION-TOF GmbH, Münster, Germany) with 25 ns pulses of 25 keV Bi+ (primary ions), bunched as ion packets with a nominal temporal extent of
